1. Field of the Invention
The present invention relates in general to exhaust emission control systems of internal combustion engines, and more particularly to the exhaust emission control systems of a type that is suitable for a diesel engine. More specifically, the present invention relates to the exhaust emission control systems of a type that includes a NOx trapping catalytic converter that traps nitrogen oxides (NOx) in the exhaust gas when the exhaust air/fuel ratio is high (viz., lean) and releases the trapped NOx from the catalytic converter when the exhaust air/fuel ratio is low (viz., rich), and a diesel particulate filter (DPF) that collects particulate matter (PM) in the exhaust gas.
2. Description of the Related Art
Hitherto, various exhaust emission control systems of the above-mentioned type have been proposed and put into practical use particularly in the field of wheeled motor vehicles powered by a diesel engine. Some of them are disclosed in Japanese Patents 2722987 and 2727906.
In the systems of these patents, there is employed such a measure that once the NOx trapping catalytic converter completes the releasing of the trapped NOx therefrom, burning of the particulate matter (PM) collected by the diesel particulate filter (DPF) starts. In other words, regeneration of the diesel particulate filter (DPF) starts after completion of regeneration of the NOx trapping catalytic converter.